Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
In many wind turbines, the generator may be electrically coupled to a bi-directional power converter that includes a rotor-side converter joined to a line-side converter via a regulated DC link. Such wind turbine power systems are generally referred to as a doubly-fed induction generator (DFIG). DFIG operation is typically characterized in that the rotor circuit is supplied with current from a current-regulated power converter. As such, the wind turbine produces variable mechanical torque due to variable wind speeds and the power converter ensures this torque is converted into an electrical output at the same frequency of the grid.
During operation, wind impacts the rotor blades and the blades transform wind energy into a mechanical rotational torque that drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally coupled to the generator so as to rotatably drive a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator. Rotational energy is converted into electrical energy through electromagnetic fields coupling the rotor and the stator, which is supplied to a power grid via a grid breaker. Thus, the main transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.
Capacitive coupling between the rotor and the stator, though not the main contributor to the generated power, can induce an undesirable rotor shaft voltage in the rotor. Under normal operating conditions, the current driven by the rotor shaft voltage is safely dissipated through a ground brush that is in contact with the rotor and ground. However, if the ground brush is not in contact with the rotor, voltage can build up on the rotor shaft. As the voltage builds up, it will discharge to ground using the lowest impedance path. For doubly-fed induction generators (DFIGs), this path corresponds to the oil in the bearing housing. As such, the voltage can discharge in a pulse, causing an arc. This cycle can be repeated as long as the ground brush is lifted, causing pitting and fluting of the bearing track which ultimately leads to bearing failure.
Thus, the present disclosure is directed to a system and method for monitoring rotor shaft voltage of a wind turbine generator to detect when ground brush lifting occurs so as to address the aforementioned issues.